This application is based upon and claims the benefit of priority of Japanese Patent Application No. H.11-366456 filed on Dec. 24, 1999, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an installation structure of an engine component (for example, a glow plug, a spark plug, injector and a volt) with a combustion pressure sensor in an engine in which the engine component is inserted into an installation hole of the engine and fixed to the engine and the combustion pressure sensor is mounted on the engine component for generating a signal representing engine combustion pressure.
2. Description of Related Art
Conventionally, as an example of the engine component with a combustion pressure sensor, a glow plug with a combustion pressure sensor to be used as an ignition aided device for starting a diesel engine, as disclosed in JP-7-139736, is known. A general construction of the conventional glow plug J1 with a combustion pressure sensor is shown in FIG. 12.
The conventional glow plug J1 with a combustion pressure sensor is comprised of a plug body 200 having a heater body 206 for generating heat when energized and a combustion pressure sensor 300 for generating a signal based on a force in responsive to combustion pressure acting on the plug body 200.
A part of the plug body 200 on an end side thereof is inserted into an installation hole 1b (plug hole, a threaded bore) formed in an engine head 1 of a diesel engine and firmly fixed to the engine head 1. A male thread 201a of a housing 201 is screwed into a female thread 1c of the installation hole 1b for fixing.
In the glow plug J1, a voltage is applied via a connecting cover 2 to a central axis 204 from an electric source (not shown). The central axis 204 is grounded via a heating coil 203, a sheath pipe 202, and the housing 201 to the engine head 1.
The heater body 206, which is composed of the heating coil 203 and the sheath pipe 202, generates heat for helping ignition of the diesel engine at its starting.
Further, the glow plug J1 has a sealing construction with which a leak of combustion gas through the installation hole 1b is prevented. As explosive combustion pressure in a combustion chamber 1a of the diesel engine is higher than that of a gasoline engine and, for example, increases up to 150 Mpa, it is very important to hold a close sealing not to leak combustion gas or pressure to outside. If leaked, the engine cannot be adequately operated due to a reduced engine output and a fluctuated engine revolution that may cause an engine vibration and a deteriorated acceleration, which a driver feels unfavorable.
To ensure the air tightness in the conventional glow plug J1, a housing taper portion 212 formed in the plug body 200 on a leading end side of the housing 201 and a taper seat surface portion 1d formed at an inner circumference of the installation hole 1b of the engine head 1 are provided to face to each other and come closely in pressurized contact with each other by screwing the housing 201 into the installation hole 1b with a recommended fastening torque for standardization (for example, 10 to 15 Nxc2x7m).
More specifically, a taper angle of the housing taper portion 212 is larger, for example, by 3xc2x0, than that of the taper seat surface portion 1d. When the plug body 200 is fastened, the housing taper portion 212 comes firmly in a circumferential line contact with the taper seat surface portion Id and the plug body 200 is fixed to the engine head 1 in a state that a circumferential part of the housing taper portion 212 in the line contact cuts into the taper seat surface portion 1d. Accordingly, the combustion gas generated in the combustion chamber 1a is prevented from leaking outside so that the sealing is ensured.
The plug body 200 so fastened can maintain the rigid fixing with the engine head 1 due to a frictional force of the housing taper portion 212 and the taper seat surface portion 1d, a frictional force of the male thread 201a of the housing 201 and the female thread 1c of the installation hole 1b, and a mutual elastic force of the housing 201 and the engine head 1.
In the glow plug J1, a ring shaped combustion pressure sensor 300 is fitted into the housing 201 at an outer circumference of the housing 201 and fixed to a surface of the engine head 1 by screwing a fixing nut 310 for the sensor 300 into the male thread 201a of the housing 201.
The combustion sensor 300 may incorporate a piezoelectric member (not shown), as shown in JP-A-139736, which converts a force acting on the plug body 200 into an electric signal (electric charge) according to an piezoelectric characteristic and outputs the electric signal representing combustion pressure.
A load is applied in advance via the housing 201 of the plug body 200 to the combustion pressure sensor 300. Combustion pressure generated in the combustion chamber 1a is transmitted via the heater body 201 and the housing 201a to the male thread 201a. Accordingly, the housing 201 including the male thread 201a is pushed upward in an axial direction of the installation hole 1b so as to deform slightly so that the load to the combustion pressure sensor 300 may be eased. A change of the load is converted into the electric signal, which is an output from a lead wire 500, so that a change of combustion pressure is detected.
However, the conventional installation structure of the glow plug J1 with the combustion pressure sensor has some drawbacks. One of the drawbacks is that a transmitting efficiency of transmitting the force from the plug body 200 to the combustion pressure sensor 300 is low and an output sensitivity of the sensor 300 is not sufficient enough.
As the plug body 200 is rigidly fixed to the engine head 1 to ensure the air tightness with the recommended fastening torque for standardization so that the housing 201 for transmitting combustion pressure is firmly retained by both of the taper seat surface portion 1d and the female thread 1c in the installation hole 1b, an amount of the deformation of the plug body 200 is remarkably limited according to a change of combustion pressure. As a result, the change of the load is limited so that the output sensitivity of the combustion sensor 300 is low.
If a problem exists in that only the sensitivity is low, the sensitivity may be easily improved in use of an electric amplifying circuit. However, in this case, as mechanical vibration noises and electric noises are also amplified at the same time, a S/N ratio is not changed. Therefore, this is not a favorable solution to cope with the problem.
An experimental test result shows that an obtained output sensitivity (a generating charge per unit pressure pC/Mpa) of an engine component with the combustion pressure sensor shown in FIG. 12 is only 5% of that of the combustion pressure sensor 300 that is not installed in the plug body and the transmitting efficiency is remarkably low.
Another drawback is that the output sensitivity of the combustion pressure sensor 300 varies largely according to engine operating conditions. A linear expansion co-efficient of the housing 201, which is a part of the plug body 200, is largely different from that of the engine head 1 because of a material difference thereof. The linear expansion co-efficient of the housing 201(steel) is 12xc3x9710xe2x88x926(/xc2x0 C.) and that of the engine head 1 (aluminum alloy) is 23xc3x9710xe2x88x926(/xc2x0 C.).
As clearly understood from the fact that there is a difference of the linear expansion co-efficient therebetween, when both of the plug body 200 and the engine head 1 receive a heating effect after the plug body 200 is fixed to the engine head 1 at a normal temperature (room temperature), the engine head 1 expands more than the plug body 200 so that the load of the plug body 200 is eased and a fastening is loosened. As an engine speed as one of the engine operating conditions is higher, the fastening is further loosened since combustion temperature is more increased.
As mentioned above, since the loosing amount of the plug body 200, that is, the load (a fastening force) of the combustion pressure sensor 300 is changed by temperature variation, there exists a difference of the output sensitivity based on the high and low engine speed conditions.
According to an another experimental test using a pressure gauge with which a change of combustion pressure is directly detected from the combustion chamber (without an influence of installation thereto) by changing the engine operating conditions from a high speed to a low speed, the test result illustrates that the difference of high and low engine speed conditions brings a 25% difference with respect to the output sensitivity (pC/Mpa) of the sensor 300.
The two problems mentioned above are applicable not only to the installation structure of the glow plug with the combustion pressure sensor but also to that of any other engine component with the combustion pressure sensor, if a part of the engine component on one end side thereof is inserted into the installation hole formed in the engine and the combustion pressure sensor is mounted on the engine component for outputting a signal representing engine combustion pressure.
A spark plug, an injector and so on are typical examples as the engine component with the combustion pressure sensor. Each of the components is rigidly retained in the installation hole to secure the air tightness and there is a difference of the linear expansion co-efficient between the engine head and the engine component.
To study more details of the above problems, FIG. 13 shows a schematic view of the conventional installation structure of the glow plug J1 with the combustion pressure sensor in the engine head 1. The combustion pressure sensor 300 has a fixing nut 310, a base seat 340 and a piezoelectric element 320 put between the fixing nut 310 and the base seat 340. A taper contact portion S indicates a portion where the taper seat surface portion 1d and the housing taper portion contact each other. The taper contact portion S hold the load (axial load) in an axial direction of the installation hole 1b generated by fastening the plug body 200 to the engine head 1.
As shown in FIG. 13, it is presumed that transmitting elements of combustion pressure are basically composed of the housing 201 of the plug body 200 (presuming that the heater body 206 is integrated with the housing 201), the nut 310 of the combustion pressure sensor 300, the piezoelectric element 320, the basic seat 340, the engine head 1 and the taper contact portion S.
FIG. 14 shows an equivalent spring system model into which the transmitting elements mentioned above is converted, while the engine head 1 is bound. Each spring constant of the transmitting elements can be shown by K=L/Axc3x97E (mm/N) where an axial length of the element is L (mm), young""s modulus thereof is E (N/mm2), and a cross section area in a radial direction is A (mm2).
Accordingly, a spring constant of the housing 201 kh, a spring constant of the nut 310 Kn, a spring constant of the piezoelectric element 320 Kp, a spring constant of the basic seat 340 Kd and a spring constant of the taper contact portion S Kt may be calculated, respectively. When combustion pressure force F is applied to the housing 201 (including the heater portion 206), a force P acting on the piezoelectric element 320, that is, a transmitting force, is shown by a following formula.
P=Fxc3x97Kt/(Kh+Kn+Kp+Kd)
The force P is represented as a function of the spring constant Kt of the taper contact portion S. Thus, as a value Kt is higher, that is, as the taper contact portion S is less resilient and more flexible and elastic, a transmitting efficiency of the combustion pressure force to the piezoelectric element 320 increases and the output sensitivity thereof increases.
According to the study mentioned above, it is understood that, as resiliency of the taper contact portion, that is, resiliency of a portion where the housing 201 comes in contact with the engine head 1 in the installation hole 1b, is lower, the transmitting efficiency of combustion pressure is increased.
An object of the present invention is to provide an installation structure of an engine component with a combustion pressure sensor to an engine in which a force is transmitted in a better way from the engine component to the combustion pressure sensor so that a sensor output sensitivity maybe improved and a change of the output sensitivity according to a change of engine operating conditions is limited.
To achieve the object, an installation hole is formed in the engine so as to penetrate from an outer surface thereof into a combustion chamber of the engine. The engine component is partly inserted into the installation hole and is fixed via an elastic member to the engine so as to come in pressurized contact therewith in an axial direction of the installation hole. With the installation structure mentioned above, the elastic member is elastically more deformable in an axial direction of the installation hole than a portion of the engine component on which the combustion pressure sensor is mounted.
It is preferable that the engine has a seat surface in the installation hole, the engine component has a contact surface to be seated via the elastic member on the seat surface, and the elastic member is a metal hollow ring whose circumference on one side thereof contacts the contact surface and whose circumference on the other side thereof contacts the seat surface. Alternatively, the elastic member may be a thin thickness portion of the engine component integrally formed therein and the thin thickness portion comes in pressurized contact with the seat the surface. Further, the elastic member may be a protruding portion of the engine protruding radially in the installation hole portion and the protruding portion comes in pressurized contact with the contact surface.
According to the conventional installation structure, the contact surface of the engine component and the seat surface of the engine are in direct and pressurized contact with and rigidly fixed to each other so that the output sensitivity is low. However, according to the present invention, force acting on the engine component in an axial direction of the installation hole responsive to combustion pressure is appropriately transmitted via the elastic member to the engine component so that the output sensitivity may be increased. Further, a gap between the engine component and the engine and a change of the load generated by a difference of linear expansion co-efficient there between are absorbed by the elastic member whose elasticity is lager. As a result, a change of the sensor output sensitivity due to a change of engine operating conditions may be reduced.